In a manufacturing line of an electric resistance welded pipe, a water hydraulic test is conducted to examine the quality of the manufactured electric resistance welded pipe, particularly the quality of a welded part called a seam part. This water hydraulic test is conducted by putting the manufactured electric resistance welded pipe of a given length between a headstock and a tailstock arranged at the front and the back of a test line, hermetically sealing the pipe at both of the front and back ends thereof, and pouring high-pressure water into the electric resistance welded pipe in this state through the headstock. The pressure of the high-pressure water reaches about 90% of assured strength. An electric resistance welded pipe having withstood this pressure for a predetermined time and not causing breakage of a welded part and not causing resultant burst of the pipe is determined to be a conforming item in terms of mechanical strength.
The following describes the outline of a procedure of the water hydraulic test. A test pipe is fixed between a headstock and a tailstock and hermetically sealed at both of opposite ends thereof. Water is poured at a low pressure (including the self weight of the water) into the test pipe from a tank through the headstock. Air in the test pipe is exhausted to the outside of the pipe through the tailstock. When the test pipe is substantially full of water, high-pressure water is supplied forcibly into the test pipe to pressurize the inside of the pipe to a required test pressure. The inside of the pipe is held at the test pressure for a predetermined time and then the pressure test is finished. After the pressure test is finished, a pressure reducing valve provided on a tail side, on a head side, or each of the tail side and the head side is opened to reduce the pressure in the pipe. Then, the test pipe is removed from between the both stocks and water in the pipe is discharged to a pit. In this way, the test is finished completely.
As described in patent literature 1, a booster mechanism using oil pressure is employed as a high-pressure water supply system of supplying high-pressure water forcibly into a test pipe. Specifically, high-pressure water is supplied into the test pipe using an oil hydraulically driven booster cylinder. More specifically, water is sucked into an output side of the oil hydraulically driven booster cylinder and then pressure oil is supplied to increase a pressure on an input side of the cylinder. This makes a piston in the cylinder advance to supply high-pressure water from the output side of the booster cylinder into the headstock and eventually, into the test pipe.
As shown in FIG. 4, a plurality of oil hydraulic pumps 1 arranged in parallel is used simultaneously as a drive system for the booster cylinder, specifically, as an oil hydraulic source from which pressure oil is supplied to the input side of the booster cylinder. The reason therefor is that, as the water pressure, the water quantity, and the supply pattern of high-pressure water to be supplied to a test pipe change in various ways in a manner that depends on the size of the test pipe, etc., the oil pressure and the oil quantity of pressure oil to be supplied to the input side of the booster cylinder are also required to cover a wide range.
Oil hydraulic control is executed using a proportional control valve 4 placed as a check valve in a secondary line 3 branching from a main line 2 extending from the plurality of oil hydraulic pumps 1 to an input side of a booster cylinder 5. Like the reason of simultaneously using the plurality of oil hydraulic pumps 1 arranged in parallel, reason of placing the oil hydraulic control valve not in the main line 2 but in the secondary line 3 branching from the main line 2 is that pressure oil to be supplied to the booster cylinder 5 is required to cover a wide range from a low pressure and a low flow rate to a high pressure and a high flow rate.
The plurality of oil hydraulic pumps 1 is a generally-used oil hydraulic unit driven by an AC motor. Thus, the oil hydraulic pumps 1 continue discharging pressure oil by rotating constantly. The pressure of the discharged pressure oil is controlled using the proportional control valve placed as a check valve in the secondary line. Specifically, if the pressure of pressure oil in the main line 2 is higher than a set pressure at the proportional control valve 4 in the secondary line 3, pressure oil is released through the proportional control valve 4 so as to maintain the set pressure. In this way, the pressure of the pressure oil in the main line 2 is maintained at the set pressure. As shown in FIG. 5, the oil pressure and the oil quantity of the oil hydraulic pump 1 are in inverse proportion to each other. The oil pressure and the pressure of high-pressure water are in proportion to each other. The oil pressure and the quantity of the high-pressure water are in proportion to each other.
In an actual water hydraulic test, a set oil pressure at the proportional control valve 4 is increased in stages such as 10 MPa, 20 MPa, and 30 MPa, for example, to shift to a set hold-on pressure finally. In response, as shown in FIG. 6, the pressure of high-pressure water is increased in stages to finally reach a hold-on pressure. The flow rate of the high-pressure water is reduced with increase in a pressure. While the hold-on pressure is maintained, this flow rate is substantially zero. At this time, in the drive system for the booster cylinder 5, much of pressure oil discharged from the plurality of oil hydraulic pumps 1 is released to the outside of the line through the proportional control valve 4 in the secondary line 3.
The following describes why the flow rate of high-pressure water to be supplied into a test pipe is reduced while the pressure of the high-pressure water is increased in stages. If a pressure at the proportional control valve 4 is set a maximum pressure corresponding to the hold-on pressure from the beginning, a pressure on the input side of the booster cylinder is increased while a high flow rate is maintained. As a result, an overshoot is caused in the pressure of the high-pressure water by the inertial force of the booster cylinder itself (inertial force of a piston), etc., to make the pressure of the high-pressure water exceed its upper limit, as shown in FIG. 7.
The hold-on pressure of the high-pressure water is set at a pressure between a pressure required for a test and an upper limit pressure. To prevent the occurrence of an overshoot in the pressure of the high-pressure water, the flow rate of the high-pressure water to be supplied into a test pipe is reduced while the pressure of the high-pressure water is increased in stages. Further, in an attempt to absorb the aforementioned inertial force completely, the set pressure at the proportional control valve is adjusted to a final pressure corresponding to the hold-on pressure immediately before the pressure of the high-pressure water reaches its hold-on pressure.
The aforementioned high-pressure water supply system using the booster cylinder and the aforementioned drive system for the booster cylinder, high-pressure water at a required pressure can be held for a required time in a test pipe. Meanwhile, increasing a pressure in the test pipe in stages to the required pressure increases what is called a cycle time. This causes an essential problem in that the number of pipes processed per unit time is increased to result in low efficiency. Additionally, an oil hydraulic pump continues rotating constantly, both in a period of increasing the pressure of the booster cylinder and in a period of not increasing the pressure of the booster cylinder. This also causes a problem in that power loss is essentially large in the pump.
Additionally, according to a trend resulting from a breakthrough technique enabling shared use of a shaping roll (patent literature 2) suggested recently, in manufacture of electric resistance welded pipes, electric resistance welded pipes that can be manufactured on one line are allowed to be increased considerably in size (pipe diameter, thickness, or length) from 8-inch diameter to 24-inch diameter or more, for example. In a water hydraulic test on manufactured electric resistance welded pipes, however, this trend causes a difference in the pressure and the quantity of high-pressure water to be poured into test pipes: a water pressure for a test pipe is several times higher than that for another test pipe and a water quantity for a test pipe is as much as 20 times larger than that for another test pipe. This in turn becomes a factor of various problems.
First, a pressure increasing speed and a change point of the pressure increasing speed are set for each difference in the size of a test pipe and each difference in a hold-on pressure. Hence, a huge volume of data should be retained to involve considerably burdensome operation for the setting. Thus, in view of increasing types in recent years manufactured on one line, conducting a water hydraulic test using one test device is considered not to be a realistic way. This causes irrationality in that many water hydraulic test devices are required for one line.
Second, as a result of a wide range of test pipe sizes, if a water hydraulic test is to be conducted using one test device, an oil hydraulic pump should conform to a maximum size. As described above, the oil hydraulic pump always continues discharging pressure oil by rotating constantly. Thus, if a test pipe size is reduced, more pressure oil is released. This causes large power loss in the oil hydraulic pump in a test period as well as in a period when the test is not conducted. Third, making the oil hydraulic pump conform to the maximum size reduces accuracy of controlling an oil pressure and an oil quantity when the oil hydraulic pump is used in a smaller size. These problems also make it difficult to conduct a water hydraulic test using one test device.
During supply of low-pressure water into a test pipe preceding supply of high-pressure water into the test pipe, air inevitably remains in the test pipe. With the air remaining in the test pipe during the supply of low-pressure water, after the supply of high-pressure water into the test pipe is started, the pressure of the high-pressure water is absorbed by compression of the air. Hence, a considerable delay is caused in pressure increase in an initial stage of water supply. This extends a time (cycle time) further required for a test. Additionally, while the air remains in an amount of about 1.5% on average, since the amount of the remaining air varies widely, this causes serious uncertainty. Thus, the remaining of the air also leads to more complicated setting operation.